I'm not seeing how to do that with Webmin either, so I'm learning firewall-cmd.
Excerpt from a bash shell script I wrote so I didn't have to remember what to do to block 1 IP address
Code:
firewall-cmd --add-rich-rule="rule family='ipv4' source address='$IP' reject"
firewall-cmd --permanent --add-rich-rule="rule family='ipv4' source address='$IP' reject"
...where $IP is passed in from the command line. The first line make an immediate change; the second line makes it survive reboots.
which is onerous...I've done a lot of web searching, too.
I'm not finding a GUI or web interface yet. Nor have I looked at the other webmin modules, yet.
NOTE: I'm on CentOS 7 so ymmv